1. Field of the Invention
The invention relates to a laser welding method that is favorable for welding plural workpieces with use of a laser beam and a weld structure.
2. Description of Related Art
For example, workpieces of two metal plates are stacked or abutted and irradiated with a laser beam for laser welding. In order to increase reliability and intensity by the laser welding, for example, in Japanese Patent Publication No. 3-80596, a technique of a laser welding method for projecting a laser beam onto a weld region with parts of surfaces of plural metallic workpieces being set as the weld region when the workpieces are welded is suggested.
In this technique, the laser beam is projected (scanned) to circle around center of the weld region. In this way, the workpieces can be melted together by being applied with a uniform amount of heat to a wide range. As a result, reliability of a weld section welded by the laser beam can be increased.
However, in the case where parts of the workpieces are melted by projecting the laser beam as described in Japanese Patent Publication No. 3-80596, heat is more likely to be dissipated from a periphery of a melted section (a molten pool) that has been melted rather than the center of the melted section, and thus solidification of the melted section starts from the periphery of the melted section and progresses toward the center of the melted section. At this time, a solidification rate from start of solidification of the center of the melted section to completion of solidification (that is, a cooling rate) is higher than a solidification rate (a cooling rate) of the periphery of the melted section.
As a result, there is a case where solidification shrinkage of the center of the weld section is completed before being compensated by liquid-phase flow and where this causes generation and extension of a crack that starts from the center of the weld section (a solidified section), in which the melted section is solidified, or the vicinity thereof. This crack extends from the center of the weld section (the solidified section), and thus a breaking mode thereof is difficult to be predicted. Especially when the workpieces made of aluminum alloy are used, such a phenomenon is significant.